1. Field of the Invention
The present invention relates to a manufacturing process, for example, of a polysilicon (p-Si) thin film transistor (TFT) liquid crystal display, and to a laser processing method and apparatus in which a workpiece such as an amorphous silicon (a-Si) film is irradiated with a pulse laser beam, and the a-Si film is poly-crystallized.
2. Description of the Related Art
A manufacturing process of a p-Si TFT liquid crystal display includes a process of poly-crystallization. The process includes: forming an a-Si thin film on a glass substrate of the liquid crystal display; and forming the thin film into a polycrystalline silicon (Si) film.
Examples of a method of poly-crystallization include a solid phase growth method and an excimer laser annealing method. The solid phase growth method includes annealing the a-Si film formed on the glass substrate at a high temperature so that a polycrystalline Si film is obtained. Since the solid phase growth method is a high-temperature process, it is necessary to use an expensive quartz glass in the glass substrate.
The excimer laser annealing method includes irradiating the a-Si film with a short-pulse excimer laser having a pulse width of about 20 ns so that the polycrystalline Si film is obtained. Since the excimer laser annealing method is a low-temperature process, mass production can be realized.
In the p-Si TFT liquid crystal display, an enhanced capability has been requested to be realized. To realize the enhanced capability, there has been a stronger demand for further enlargement of a current crystal particle diameter of the polycrystalline Si film. Concretely, the crystal particle diameter is around 0.5 μm in the current method, and there has been a strong demand for setting of the diameter to several micrometers or more.
Reasons for this will be described. There is a numeric value of mobility as a factor which influences the capability of a semiconductor device. The mobility represents a movement speed of an electron. The movement speed drops, when the crystal particle diameter is small and there are many crystal particle fields in a path of the electron. When the movement speed decreases, the enhanced capability of the semiconductor device cannot be expected. Therefore, there is a demand for enlargement of the crystal particle diameter of the polycrystalline Si film.
Examples of an enlargement method of the crystal particle diameter include techniques described, for example, in Jpn. Pat. Appln. KOKAI Publication No. 56-137546 and PCT National Publication No. 2000-505241. The Jpn. Pat. Appln. KOKAI Publication No. 56-137546 discloses a method for using a roof-shaped laser beam to scan a work. The PCT National Publication No. 2000-505241 discloses a method called super lateral growth.
These methods include: successively irradiating an Si thin film with a laser beam having a linear or roof-shaped pattern in synchronization with movement of the Si thin film, that is, the glass substrate. The present inventors have verified that the crystal particle diameter of the polycrystalline Si film is enlarged by these methods.
However, in these methods, since the Si thin film is successively irradiated with the laser beam at an interval, the glass substrate has to be moved for each irradiation with the laser beam. A movement distance needs to be between about 0.1 μm and 1.0 μm.
Therefore, in order to form the Si thin film into the polycrystalline Si film on a large-sized glass substrate, for example, a 300 mm×400 mm glass substrate, the glass substrate has to be moved at an interval of about 0.1 μm to 1.0 μm. To form the polycrystalline Si film over the large-sized glass substrate, a throughput of several hours is required, and formation cannot be realized.
Examples of a method for forming the polycrystalline Si film at a higher speed include a method described in Jpn. Pat. Appln. No. 9-217213. This method includes: forming a plurality of repeated patterns 1 on a mask as shown in FIG. 1; and moving the glass substrate by a pitch of the pattern 1.
Subsequently, the glass substrate is irradiated with the laser beam through the mask. In a region irradiated with the laser beam, a crystal grows, and the whole region irradiated with the laser beam is poly-crystallized. FIG. 1 shows a crystal grown region 2.
Subsequently, the glass substrate is moved in a stepwise manner by the region irradiated with the laser beam.
Subsequently, the glass substrate is irradiated with the laser beam through the mask. In the region irradiated with the laser beam, the crystal grows, and the whole region irradiated with the laser beam is poly-crystallized.
Thereafter, the irradiation with the laser beam and the stepwise movement of the glass substrate are repeated, and the whole glass substrate is poly-crystallized.
There is another method for forming the polycrystalline Si film at a high speed. In the method, the pitch of the pattern 1 formed on the mask is reduced as shown in FIG. 2, and the glass substrate is not moved. This mask is used to grow the crystal in a region portion irradiated with the laser beam.
The method includes: using the mask with the repeated pattern, for example, having a pattern width of 2 μm and pitch of μm formed thereon to form, for example, a polycrystal having a length of 2 μm and width of 0.3 μm.
However, the former method requires a throughput of several hours, is unrealistic, and has a low productivity. In this method, when a width of the laser beam is set, for example, to 5 μm or more as shown in FIG. 3, a heat gradient of a middle portion in the region irradiated with the laser beam is reduced.
Therefore, boundaries of opposite ends of the region irradiated with the laser beam have a large particle diameter, but the middle portion is micro-crystallized. Then, a transistor is formed on the crystallized region, but the micro-crystallization forms an Si crystal film which inhibits enhancement of the capability of the transistor.
In the latter method, there are large influences of a stop operation in a substrate conveyance system for moving the glass substrate in the stepwise manner, deceleration operation during restart, and an acceleration time. Therefore, the throughput in an actual mass production line is not achieved, and a further high-speed processing is necessary.
In the latter method for narrowing the pitch of the pattern 1, because of a heat influence from the adjacent pattern 1, a growth speed of a lateral direction (vertical to a film thickness direction) of the Si film is lowered. Therefore, in the latter method, a part of the region irradiated with the laser beam, for example, the middle portion of the irradiated region is micro-crystallized, and a micro crystal region 3 is formed as shown in FIG. 2.
Furthermore, when the pitch of the repeated pattern 1 is narrowed, the whole surface of the region irradiated with the laser beam is micro-crystallized, and the mobility of the electron is lowered as shown in FIG. 4.